The present invention generally relates to mixture compositions and more specifically to cementitious compositions.
Cementitious compositions are utilized as binders in various materials. One type of cementitious composition is Portland cement, which is manufactured for use, among other things, as a binder in concrete. The manufacture of Portland cement generally requires large-scale production facilities, and the process requires heating limestone to 2000 degrees in gas powered kilns. Portland cement production is estimated to release into the atmosphere about one pound of carbon dioxide for every pound of Portland cement produced.
Coal power plants produce as much as fifty percent of the electrical power consumed in the United States. The Portland cement production process and the coal-burning electrical power generation process produce carbon dioxide as a waste product. Coal burning power plants are the single biggest producers of carbon dioxide, collectively. In addition to the considerable carbon dioxide production, coal-burning power plants produce other byproducts such as fly ash. It is estimated that coal-burning power plants produce over 80,000,000 tons of ash per year, most of which is placed in some form of landfill, or lagoon. In the United States, it is estimated that approximately two thirds of coal combustion byproducts are simply disposed of, generally in landfills, and environmental agencies are concerned about proper disposal.
Landfills are overflowing with ash, figuratively and literally. On Dec. 22, 2008, in Kingston, Tenn., a Tennessee Valley Authority power plant ash lagoon broke, spilling 5.4 million cubic yards of ash and sludge. The cleanup was projected to cost $500-$800 million. In January 2009, Constellation Energy settled for $54 million to address issues caused by their use of a gravel pit to dispose of unused fly ash. The Environmental Protection Agency estimates that there are nearly 1300 unregulated ash ponds or lagoons. Due in part to the Tennessee spill and the environmental stance of the new administration, the EPA has promised to introduce new regulations governing the disposal of coal-combustion by-products by the end of 2009. Also, to absorb the relative risk of such catastrophes, disposal costs have been rising and will be certain to rise dramatically as traditional, unregulated options must be phased out. Current disposal costs are generally about thirty to fifty dollars per ton of ash.
Instead of simply disposing of ash in a landfill, the ash may be put to some use. However, utilization of fly ash, such as in concrete, is generally low. It is estimated that only about ten percent of all of the 80,000,000 tons of ash per year is used in concrete. The reasons for such low utilization in concrete are varied. One reason is the relative lack of consistency of generated fly ash. Presently, most power plants are optimized to run on the lowest cost per megawatt of power generated, while keeping in mind all regulatory mandates. To reduce cost of production, power plants may buy a wide variety of coal to obtain low prices on the spot market. Further, power companies often contract with third party recycling companies to dispose of or market their byproducts. All of these issues create variations in the quality and usability of the byproducts. In addition, though power plants produce ash generally constantly, much of the construction industry is seasonal, especially in northern climates. Thus, while there are other uses for fly ash, such as soil stabilization, flowable fill, asphalt additive, and traffic base, when concrete demand slumps, generated ash is generally not utilized, and must be disposed of. Indeed, these alternate uses further complicate any effort to track where the ash is being used, and whether the ash is being used in an application thereby reducing a carbon footprint, such as by replacing some Portland cement.
Portland cement has been used in concrete for nearly two centuries. However, the concept of using some form of ash in a cementitious composition is an even older technique. Indeed, Roman cement, used in such buildings as the Pantheon in Rome, built around 123 A.D. featuring a 142 foot dome including neither rebar nor Portland cement, is thought to have employed a combination of two parts volcanic ash and one part lime. The cementitious composition used in constructing the Hoover dam employed the addition of fly ash from coal fired power plants for added strength. The Sears Tower (now the Willis Tower) in Chicago is estimated to have approximately thirty percent fly ash in its cementitious composition and the Dubai Tower is estimated to have approximately forty percent fly ash in its cement.
Though ash has been used in cement since Roman times and has even been used where high strength is necessary, there is a perception that it adulterates Portland cement. This perception has some basis in fact. Indeed, prior attempts have been concerned with determining an amount of fly ash that may be added to replace a minority portion of Portland cement in a cementitious mixture, rather than substantially replacing it. In such a case, there is usually a point of diminishing returns, and the addition of additional fly ash will retard and/or weaken the resulting cement. As mentioned earlier, coal fired power plants are designed and run to generate inexpensive electricity; they are not optimized for production of consistent, high-quality fly ash. Consequently, fly ash can vary in quality even from the same plant burning different source coal as well as different plants burning the same coal. To temper variations, some regulatory agencies have enacted specific rules. For instance, if fly ash is to be used in road construction in Texas, the Texas Department of Transportation requires that the ash must be supplied from the same source for an entire project. However, as a plant shuts down or starts up, its coal burning effectiveness and its ash can change.
In addition to coal and plant variations, the actual coal-burning process, which must be carried out to meet air quality regulations, may affect the quality of the resulting ash. For instance, regulations that limit the amount of nitrous oxide that can be produced also indirectly usually reduce the temperature at which coal is burned at a given power plant, thus leaving some coal in its unburned state. This loss of ignition (LOI) carbon is captured with the fly ash, and it significantly reduces the effectiveness of air entrainment chemicals used in Portland cement, which are used to improve its strength in a freezing and thawing environment. LOI carbon of even 3% in a cementitious mixture comprising only 20% ash can still significantly reduce the air entrainment of the concrete.
Though there may not be a current viable method for reducing carbon dioxide emissions of coal-fired power plants, the possibility exists for using the byproducts thereof to create an alternative to Portland cement. The net effect would be a considerable reduction in aggregate carbon dioxide generation. On the other hand, if coal-fired power plant byproducts continue to be simply relegated to a land fill, no beneficial reduction in carbon dioxide emissions will occur.
Consequently, if more fly ash that is produced by coal-burning power plants can be put to a constructive use, despite its variability, less fly ash will be subject to disposal in landfills. Also, to the extent that cementitious compositions incorporating fly ash can be used as a replacement for Portland cement, such use will reduce the amount of carbon dioxide released by Portland cement production.